Not all chins get along with all other chins, so they might or might not get along, some say it's about a 50/50 chance. Also keep in mind that bonding can take anywhere from days to years, assuming it ever happens. Chins are social herd animals so even if they don't end up getting along having another around can be beneficial, like living alone but having a neighbor vs living alone in isolation. With a single chin you just have to remember that you are their only socialization, so they need you to interact with them everyday for their mental well being.
A single unit is an ok size for a single chin if things don't work out, but you will need another cage at least at first though, new chins should be quarantined for 30 days before even being in the same room as your current chin. That gives the new chin time to settle in, for any illness to show up before exposing your current chin and for you to take the chin in for a wellness check, as well as it gives time for you to start forming a bond with the new chin. Once the new chin is all in the clear then you can move it to the same room. The way that seems to work the best is slowly move cages closer, once they are good side by side then you can introduce them in neutral territory for playtime and go from there. Neutral territory is even more important with females since they tend to be more territorial then males.